Experimental Modelling of Debris Damming Loads on Structures in Extreme Flooding Events

Abstract

Extreme flooding events have resulted in significant loss of life and extensive damage to coastal communities. These events include tsunamis, hurricanes, flash floods, and tropical storms. The devastation and destruction caused by them has been illustrated recently in the 2004 Indian Ocean Tsunami, the 2005 Hurricane Katrina, and the 2011 Tohoku Tsunami. The unexpected failure of infrastructure as a result of large-scale flood events demonstrated that past design guidelines did not properly address the various loads occurring during these events, particularly in terms of extreme loads. Research on extreme loading on infrastructure due to debris has focused predominantly on the impact of individual debris objects colliding with structures. Little research has been conducted on the loading due to debris accumulating onto or around structures, commonly referred to as debris damming. Debris dams form when entrained solid objects accumulate in-between or onto the face of structures. Debris damming has been treated as a quasi-static load, which can increase hydrodynamic loading due to an enlarged cross-sectional area of the structure exposed to the flow. In addition, debris damming has been shown to result in overtopping of structures and increased scour around them. This research aimed to investigate the impact of debris dams on structural loading and surrounding flow behaviour. This study examined the influence of debris dam shape, porosity, and height on loads induced onto structures. In addition, force coefficients were determined and analyzed in terms of Froude number and debris dam geometry. Experiments were conducted in the Hydraulic Flume at the University of Ottawa, Canada. Idealized debris dams were attached to a column and tested in both steady-state and transient flow conditions. The experimental program conducted in steady-state conditions was tested with flows exhibiting Froude numbers ranging from 0.37 to 0.61 and the program conducted in transient flow conditions, through the use of a dam-break wave, tested Froude numbers ranging from 0.92 to 1.56. Due to the random nature of debris dam formation, semi-cylindrical and rectangular debris dams with three varying heights and three porosities were tested. The experimental campaigns investigated the influence of debris dam geometry on structural loading and the surrounding flow behaviour. The research conducted showed that the loading due to debris damming increased with water depth and flow velocity. The resistance coefficients were found to be a function of the Froude number and the flow depth to obstacle width ratio. Increased dam height resulted in increased resistance forces acting on the debris dams and columns. However, the height had greater impact on the resistance coefficients in steady-state flow conditions comparing to that observed in the tests conducted in transient flow. The porosity resulted in a decrease in loading; however, the porosity had little influence on the effective resistance coefficients as porous holes result in an increase in skin friction.